1. Field of the Invention
This invention relates to a novel hollow fiber of a high molecular weight compound, and particularly to a hollow fiber having selective permeability which makes it useful for blood dialysis, ultrafiltration, reverse osmosis, and the like. The method involves jointly extruding a spinning solution of the high molecular weight compound about an acid or alkaline reacting salt solution core whereby controlled hydrolysis of the extruded high molecular weight compound is achieved.
2. Description of the Prior Art
Recently, semipermeable hollow fibers have been used for blood dialysis, for fluid separation based upon the principle of ultrafiltration, and for reverse osmosis.
Hollow fibers should have a uniform cross section since when such a fiber is used, for example, for blood dialysis the pressure in the inerior of the hollow fiber usually exceeds that at the exterior of the hollow fiber. If the hollow fiber is not uniform in thickness, there exists the possibility that the fiber may be broken at its thinnest part. Also, it is known that when the cross section of the hollow fiber is significantly non-circular, blood is apt to coagulate in the hollow fiber.
When the hollow fiber is used for reverse osmosis, high pressures of up to 100 atmospheres may be applied to the outside of the hollow fiber. In this instance, it is necessary that the cross section of the hollow fiber should be close to a true circle and that the thickness of the wall of the hollow fiber should be uniform. Otherwise, the hollow fiber may become significantly deformed. When the cross section of the hollow fiber is not uniform, the polarized concentration due to nonuniform flow substantially reduces the permeability performance of the fiber.
In a known method of producing hollow fibers, a spinning solution or dope is extruded from an annular slit of a double pipe orifice to form a sheath solution and simultaneously, a gaseous or liquid fluid is extruded through the inside pipe of the double pipe orifice to form a core fluid. Where a gaseous material such as an inert gas is used as a core fluid, the resulting hollow fiber is apt to be crushed because the direction of the extruded filament after being spun is usually changed by means of a guide bar in the coagulation and washing bath. When the moving filament is pressed on the guide bar with appreciable force, it is quite often deformed to a flat configuration which is unsuitable.
These disadvantages are reduced through the use of a melt spinning procedure. However, it is difficult by means of this procedure to produce a hollow fiber having a favorable selective permeability.
A wet spinning process has been proposed to overcome these difficulties and to provide a wide range of controllability. In the wet spinning process, the spinning conditions such as spinning solution concentration and coagulation bath conditions can be varied widely. However, it is difficult to produce a hollow fiber of a uniform shape by this process, and the production rate is usually very low because of the low speed of spinning.
In the case where the core being extruded is liquid instead of gaseous, the deformation of the moving filament at the guide bar is considerably avoided. However, another problem may be developed which is a rapid coagulation of the sheath by the instantaneous diffusion of the core liquid into the sheath immediately after being spun. Specifically, a thin skin layer is first formed on the inner interface of the spun sheath in contact with the core liquid, and another thin layer is formed on the outer surface of the spun sheath by contact with the coagulation bath liquid when the moving filament is introduced into the coagulation bath. These two layers determine the dimensional configuration of the hollow fiber, and subsequent coagulation between the two layers usually develops numerous macro-voids in the membrane of the hollow fiber. These voids serve to scatter light and as a result the resultant hollow fiber looks whitish. Selective permeability cannot be obtained from such a fiber because the membrane has unfavorable macro-voids instead of the desired micro pores. Furthermore, spinnability is very poor because of the rapid coagulation of the dope immediately after being spun. The hollow fiber can be obtained only at low speeds, at most up to 15 m/min, and has very poor mechanical properties. Also, the skin layers formed on both the inner and outer surfaces of the hollow fiber lower the permeability of the fiber.
A dry jet-wet spinning method in which the dope is extruded into a gaseous space and is then introduced into a coagulation liquid is considered to be preferable for producing hollow fibers having suitable permeability. In this method, a spinning dope is extruded from an annular slit to form a sheath dope, and the extruded sheath dope passes through a gaseous space before being introduced into a coagulation bath. The gaseous space may be occupied by an inert gas or air, or it may contain the vapor of the solvent of the spinning dope. The core liquid is preferably non-coagulative at least before the spun dope is introduced into the coagulation bath. For example, in the production of hollow fibers of cellulose by the cuprammonium process, liquids such as benzene, toluene, trichloroethylene, n-hexane, or perchloroethylene, which are miscible with water are employed as core liquids. With this method, the organic core liquid must be removed from the hollow fiber later in the process, and this involves long troublesome operations. Furthermore, from the standpoint of pollution problems, the use of toxic organic solvents is to be avoided.
When the hollow fiber is used for medical purposes such as in blood dialysis and for any process involving foods, for example, concentration of juices, the toxic organic materials must be completely washed out from the hollow fiber.